Unless I missed a post, all we know is the OP has a small corroded area on his machine which is probably growing, but whose rate of change is below naked eye obvious from sample to sample.
We don’t know what causes it. We don’t know if it’s growing. We don’t know if the growth is in diameter, depth, or both. We don’t know if it’s growing due to abrasion, then corroding due to lack of lube or damage to the anticorrosive coating. Or if the corrosion is a self-sustaining chemical process eating the substrate. Or is some electrodynamic process. Or is the result of a leak of a corrosive chemical used in the process.
The OP may know some of this, or may at least have a working theory. But IMO we don’t and IMO shouldn’t be assuming any specific cause or observable side effect.
So OP, please give us something more to go on. We might actually get someplace useful for you.
A 2-part liquid silicone molding compound would probably be the most likely to work and give the best surface definition. You could cast a dollop of epoxy into the silicone negative to get the actual surface profile.
If you know that you can pry it off after it hardens, I also like the epoxy putty idea.
Like the Cerrosafe material, McMaster-Carr sells several low melting point metals that are used for fixturing odd shaped parts. Some melt in boiling water or with a heat gun.
How about a digital photograph of a precision scale lying across the damaged area?
Yes, I have a small corroded area on my machine. It has a big part that is special and difficult to replace and made of a metal that oxidizes, and this part is protected by electroplating. However, the electroplater did something… funny… to certain dime sized locations, and now the plating has failed in these locations and fallen away in big flakes. So now those particular locations no longer have protection. The metal oxidizes the way iron does, by forming an oxide that is loose and porous and forms big flakes that fall off and expose more metal, as opposed to the way aluminum does, which is self limiting and seals out the oxygen. Thus, in principle, it should eventually oxidize throughout and be nothing but dust on the floor. Expensive dust.
In principle the corroded spots should be growing, in depth as well as radially. The exposed metal is this round area, part of a big flat surface. Picture that you painted a block of iron but left a disk of masking tape on it, and then pulled the tape off, exposing that round spot. I guess it would be spreading laterally under the paint at the same rate it is going perpendicularly deeper into the surface over time. I don’t know how fast they are growing, and that’s why I’d like to make replicas. Should I picture that these defects are going to be a millimeter deep in a couple years, or an inch deep, or what? When they’re an inch or two deep I think they’ll cause a malfunction.
I know about brush plating. That’s part of what the plater did in these spots that was “funny”. It didn’t hold up long enough to be useful.
Sorry to be all secretive. It’s a proprietary gig, not my secrets to give away. Any specific details that would help?
What’s nice about a cast of the surface is that it could be studied by many means, including placing the cast into some measurement instrument. These spots are somewhat inaccessible but perhaps depth gages would work well enough; actually as far as ordering something from McMaster goes, depth gages are probably the quickest way to get something kinda successful.
I know the low temp casting metals, Cerrosafe and whatnot. They have very little viscosity, and the local geometry won’t let me hold them in place. A clay or putty would work, and maybe a silicone, depending on if it’s runny.
Could be that the time circuits are leaking future-ocity out in a few spots and this is causing the metal to age faster where it the time leakage is pooling. Don’t get any of that stuff on you.
Depth gauges will surely suffice - you could probably make some little ones that are little more than a horseshoe shaped piece of metal with a fine threaded screw down through the middle straddle the legs across the defect, turn the screw down until it touches the bottom of the dip, then superglue the thread - when you next measure the defect, there will be a gap beneath the tip of the screw if the defect is deeper.
McMaster is great but they are high on their measuring tools for your purposes.
May I suggest a import dial depth gage with a point tip. A normal contact tip won’t reach to the bottom of most pitting.
An old toolmaker trick is to use Silly Putty for getting positive forms of hard to measure features.
Thanks, these instruments look like a good approach. By the way, Silly Putty is polydimethylsiloxane, or silicone. Whether to let it be runny or stuff and whether to crosslink it into a rubber, well, that’s the rub.
You seem concerned about the cost of materials from McMaster Carr and seem to be seriously considering Silly Putty. Yes, there are cheaper sources and I adapt technolgies across disciplines as well…
How valuable is the machine you’re trying to save?
Alternately, is it something you can monitor by applying some paint or marker pen before going to all the complexity of a moulding or detailed surface model?
If the area is painted with a continuous coating, then next time you look you may see any abrasion / wear / bare metal. It also means you can take photos and gradually track rate of surface loss, every time the spot becomes accessible.
I’m not concerned about McMaster. I love those people. Are you confusing me with MoldMonkey?
I did. They fixtured the work with washers in these locations. They say they won’t do it again but that doesn’t help the present machine.
It’s not abrasion or wear. It’s oxidation. The part is too hot for paint, which is why it’s plated. If I could coat these spots with something that stopped the oxidation, the part would be truly fixed, not just easy to monitor. The combination of this metal and this temperature sets the stage for a problem, and we deal with this problem frequently (this particular machine is just the latest example).
